1. Field of the Invention
The present invention relates to an improvement in contour compensating circuits (i.e. horizontal edge emphasizing circuits) to be incorporated in a video camera, for instance, in order to compensate aperture distortion of a scanning beam spot within an image pickup tube.
2. Prior Art
FIG. 1A shows an example of prior-art contour compensating circuit composed of an input terminal 1, an amplifier 2, a .DELTA.t delay line 3, a comparator 4, and an output terminal 5. When a video signal (e.g. brightness signal) generated from an image signal generating element (e.g. image pickup tube) as shown by (A) in FIG. 1B is inputted to the input terminal 1, this signal is amplified by the amplifier 2 and then applied to the .DELTA.t delay line 3 via an impedance R to delay the amplified signal by a timer period .DELTA.t, as shown by (C) in FIG. 1B. This delay-line output signal is reflected at point C in FIG. 1A due to impedance mismatching between the delay line 3 and the comparator 4 and therefore returned to point B with a delay time .DELTA.t through the same delay line 3, as shown by (B) in FIG. 1B. At this point B, the reflected signal will not be further reflected due to impedance matching between the impedance R and the delay line 3. Therefore, the signal waveform at point B can be represented as (B) in FIG. 1B and that at point C can be represented as (C) in the same figure. These two signals as shown by (B) and (C) are compared by the comparator 4, and a comparator output signal (i.e. differential signal) as shown by (D) is outputted from the output terminal 5 as a contour compensating signal (i.e. edge signal). This edge signal is added to a main signal transmission line (not shown) connected to point C in FIG. 1A to perform contour compensation or edge emphasization.
The frequency characteristics of the above-mentioned contour compensating circuit can be expressed in the same way as in cosine-filter characteristics in digital transmission as follows: EQU F.sub.1 (.omega.)=1/2(1-cos.multidot..omega.)
These characteristics can be represented by solid curve shown in FIG. 1C, in which the peak frequency .omega..sub.0 is dependent upon the delay time of the delay line 4. For example, if .DELTA.t=200 ns, f.sub.0 =.omega..sub.0 /2.pi.=2.5 MHz.
In the prior-art contour compensating circuit, however, there exists a problem in that the frequency characteristics are fixedly determined and therefore unadjustable as shown by the solid curve in FIG. 1C. In other words, when the output signal is required to be emphasized only within the high frequency range, since the output signal is simultaneously emphasized even within the medium frequency range, it is impossible to obtain required frequency characteristics as shown by dashed curve in FIG. 1C. Further, there exists another problem in that the peak value is also unadjustable because the delay time of the delay line is fixedly determined.